AC coupled data communication links are useful for example in applications where the capability of accommodating different DC bias or common mode voltages for transmitting and receiving circuits is desired. A common approach to implementing AC coupled links is to place a series-connected AC coupling capacitor, also referred to as a AC blocking capacitor, along the transmission line path between transmitting and receiving circuits to act as a high-pass filter.
However, such AC coupled data links can be susceptible to a phenomenon known as DC or baseline wander. For example, when a significantly unbalanced number of binary ‘ones’ and ‘zeros’ are transmitted over an AC coupled link, a voltage droop can occur at the receiver side of the AC coupling capacitor. Such signals include a significant low-frequency or DC component that is blocked by the AC coupling capacitor. Voltage droop can accumulate over time and affect the vertical margin on the received signal and hence the link margin.
For example, the paper “A 32 Gb/s Backplane Transceiver with On-Chip AC-Coupling and Low Latency CDR in 32 nm SOI CMOS Technology,” Gangasani et al., IEEE Journal of Solid-State Circuits, 2014, discusses the use of on-chip AC coupling capacitors in order to eliminate impedance discontinuities and signal quality degradation associated with on-board AC coupling capacitors. However, the small sizes of such on-chip capacitors is recognized as a potential source of baseline wander, in which long runs of ‘ones’ or ‘zeros’ can induce a transient shift on the low-frequency component of the received signal after the AC-coupling capacitors, which can degrade the receiver sensitivity.
Various solutions for mitigating DC wander have been proposed in the literature. For example, several solutions such as “Decision-Feedback Restore” and “Passive Feed-Forward Restore” are discussed in the above-mentioned paper by Gangasani et al. However, these and other solutions can require large resistive or capacitive filter components, relatively high circuit complexity, performance limitations and trade-offs, and the like.
Therefore there is a need for a circuit apparatus for mitigating DC wander in an AC coupled link that is not subject to one or more limitations of the prior art.
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